linux-kernel - Re: epoll: possible bug from wakeup

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Date:	Fri, 8 Mar 2013 20:49:44 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	NeilBrown <neilb@...e.de>, "Rafael J. Wysocki" <rjw@...k.pl>,
	linux-kernel@...r.kernel.org,
	Davide Libenzi <davidel@...ilserver.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: epoll: possible bug from wakeup_source activation

Arve Hjønnevåg <arve@...roid.com> wrote:
> On Thu, Mar 7, 2013 at 5:30 PM, Eric Wong <normalperson@...t.net> wrote:
> > Eric Wong <normalperson@...t.net> wrote:
> >> Hi Arve, looking at commit 4d7e30d98939a0340022ccd49325a3d70f7e0238
> >> (epoll: Add a flag, EPOLLWAKEUP, to prevent suspend ...)
> >>
> >> I think the reason for using ep->ws instead of epi->ws in the unlikely
> >> ovflist case applies to the likely rdllist case, too.  Since epi->ws is
> >> only protected by ep->mtx, it can also be deactivated while inside
> >> ep_poll_callback.
> >>
> >> So something like the following patch might be necessary
> >> (shown here with extra context):
> >>
> >> --- a/fs/eventpoll.c
> >> +++ b/fs/eventpoll.c
> >> @@ -968,39 +968,45 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k
> >>       if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
> >>               if (epi->next == EP_UNACTIVE_PTR) {
> >>                       epi->next = ep->ovflist;
> >>                       ep->ovflist = epi;
> >>                       if (epi->ws) {
> >>                               /*
> >>                                * Activate ep->ws since epi->ws may get
> >>                                * deactivated at any time.
> >>                                */
> >>                               __pm_stay_awake(ep->ws);
> >>                       }
> >>
> >>               }
> >
> > Thinking about this more, it looks like the original ep->ovflist case of
> > using ep->ws is unnecessary.
> >
> > ep->ovflist != EP_UNACTIVE_PTR can only happen while ep->mtx is held (in
> > ep_scan_ready_list); which means ep_modify+friends cannot remove epi->ws.
> >
> 
> The callback function in ep_scan_ready_list can call __pm_relax on it though.
> 
> > ep_poll_callback holding ep->lock means ep_poll_callback prevents
> > ep_scan_ready_list from setting ep->ovflist = EP_UNACTIVE_PTR and
> > releasing ep->mtx.
>
> This code is reached when ep_scan_ready_list has set ep->ovflist to
> NULL before releasing ep->lock. Since the callback function can call
> __pm_relax on epi->ws without holding ep->lock we call __pm_stay_awake
> in ep->ws here (the callback does not call __pm_relax on that).

Thanks for the explanation.  I got "deactivate" and "destroy"
mixed up.  However, I'm still concerned about the "destroy" case:

> >
> >>               goto out_unlock;
> >>       }
> >>
> >>       /* If this file is already in the ready list we exit soon */
> >>       if (!ep_is_linked(&epi->rdllink)) {
> >>               list_add_tail(&epi->rdllink, &ep->rdllist);
> >> -             __pm_stay_awake(epi->ws);
> >> +             if (epi->ws) {
> >> +                     /*
> >> +                      * Activate ep->ws since epi->ws may get
> >> +                      * deactivated at any time.
> >> +                      */
> >> +                     __pm_stay_awake(ep->ws);
> >> +             }
> >>       }
> >
> > I still think ep->ws needs to be used in the common ep->rdllist case.
> 
> ep_scan_ready_list calls __pm_relax on ep->ws when it is done, so this
> will not work. ep->ws is not a "ep->rdllist not empty wakeup_source is
> is a "ep_scan_ready_list is running" wakeup_source.

What happens if ep_modify calls ep_destroy_wakeup_source
while __pm_stay_awake is running on the same epi->ws?
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